LPB Application Note INCONEL 718 LOW CYCLE FATIGUE by klutzfu59

VIEWS: 0 PAGES: 2

									LPB Application Note:

INCONEL 718 LOW CYCLE FATIGUE

RESIDUAL STRESS (MPa)

SET is applying Low Plasticity Burnishing (LPB) to significantly improve low cycle fatigue (LCF) capabilities for engine components. The threat of LCF failure greatly increases the cost of inspection, RESIDUAL STRESS DISTRIBUTION DEPTH (10 in.) maintenance, and operation of turbine engines. 0 20 40 60 400 Surface enhancement, the introduction of a layer of compressive residual stress, is one of the few practical and affordable ways of improving the LCF 0 0 performance of engine components. LPB has demonstrated spectacular improvements in LCF for -400 Inconel 718 at turbine operating temperature of 1000° F through the introduction of deep compressive -100 -800 residual stresses.
-3

RESIDUAL STRESS (ksi)

LPB has been demonstrated to produce compression of far greater depth and magnitude than shot peening. -1600 Additionally, LPB is applied using conventional CNC 0 400 800 1200 1600 machine tools in a manufacturing environment. LPB DEPTH (x10 mm) 40% PERCENT COLD WORK DISTRIBUTION can produce compression exceeding 0.040 in. in 20 turbine engine alloys such as IN718, as shown in Figure 1. In recent studies supported by NASA, 10 variations in crack shape (aspect ratio, a/c) resulting from processing-induced residual stress distributions in IN718 were observed post-test via heat tinting method. 0 0 400 800 1200 1600 Two examples of heat-tinted crack surfaces, DEPTH (x10 mm) photographed post-test, are presented in Figure 2. The depth of compression for LPB far exceeds that Figure 1: Residual stress in IN718. achievable by shot peening, and the amount of cold work can be lower than shot peening and laser shocking. Controlled low cold working provides stability of compressive stresses at elevated temperature and in the event of mechanical overload as may occur during operating conditions for turbine components.
-1200 -200
-3

8A SHOT PEEN GRAVITY PEENED LASER SHOCKED, 3X HIGH LPB

PERCENT COLD WORK

-3

Figure 2: Heat Tinted Fracture Surfaces of Kb Specimens, shot peened on the right, LPB on the left.

0.17 in.

S-1004

IN 718 Kb Crack Growth, 90 ksi

LPB can be applied with CNC machine tool controlled single point tools to disk bores, web regions, and with special tooling to dovetail slots. Caliper tooling provides processing of both sides of thin blade sections simultaneously. CNC controlled caliper tooling has been applied successfully to fan and compressor blades for both aircraft engine compressors and ground based power generation turbine blades. LCF strength can be increased 400-700% over shot peening after elevated temperature exposure because of the stability of the compressive stresses provided by LPB. The deep high magnitude compressive layer has been shown to dramatically reduce crack growth rates at elevated temperatures of 1000° F. Results of crack depth, (a), and crack length, (2c) vs. cumulative cycles from selected specimens are graphically presented in Figures 3 and 4. The reduction of crack growth rate shown for LPB specimens over shot peened and as-machined specimens is dramatic. Obvious also was that life increased with increasing LPB intensity. Lives for high intensity LPB applied parallel to the crack plane exceeded those for high intensity LPB applied perpendicular to the crack plane. Lives for high intensity LPB applied perpendicular to the crack plane were at least comparable to lives for medium intensity LPB parallel to the crack plane. Fatigue life results for various IN718 specimens are shown in bar chart form in Figure 5. From the
High
5

5.000
As-Machined + 600C/100 hrs Shot Peened + 600C/100 hrs High LPB + 600C/100 hrs Med LPB + 600C/100 hrs Low LPB + 600C/100 hrs

0.20

4.000 CRACK DEPTH, a (mm)

0.15

3.000 0.10 2.000 (in.) (in.)

0.05 1.000

0.000 0.0 5.0x10
4

1.0x10

5

1.5x10

5

2.0x10

5

2.5x10

5

0.00 5 3.0x10

CYCLES

Figure 3: Surface Crack Depth vs. Cycles for Various Surface Treatments after Thermal Exposure.
IN 718 Kb Crack Growth, 90 ksi
10.000 9.000 8.000 CRACK LENGTH, 2C (mm) 7.000 6.000 5.000 4.000 3.000 0.10 2.000 1.000 0.000 0.0 5.0x10
4

0.40
As-Machined + 600C/100 hrs Shot Peened + 600C/100 hrs High LPB + 600C/100 hrs Med LPB + 600C/100 hrs Low LPB + 600C/100 hrs

0.35 0.30 0.25 0.20 0.15

0.05 0.00 5 3.0x10

1.0x10

5

1.5x10 CYCLES

5

2.0x10

5

2.5x10

5

Figure 4: Surface Crack Length vs. Cycles for Various Surface Treatments after Thermal Exposure.

2.0x10

High

CYCLES TO FRACTURE

High

results of this study, LPB applied to engine components offers significant improvement in anticipated fatigue life at nominal cost.
High

1.5x10

5

Medium

1.0x10

5

5.0x10

4

Low

0.0

Figure 5: Cyclic Lives for Specimens Processed and Thermally Exposed after Fatigue Pre-cracking.

As-Machined

Shot Peen

LPB Parallel to Crack Plane

LPB Perpendicular to Crack Plane

Complete text papers describing turbine engine sustainment applications of LPB are available at www.surfaceenhancement.com. Surface Enhancement Technologies 5521 Fair Lane, Cincinnati, OH 45227 www.surfaceenhancement.com Telephone: 513.561.1520 Facsimile: 513.561.0886

S-1004


								
To top